Human umbilical vein endothelial cells (HUV-EC) proliferate in a growth factor-rich environment (GFRE) consisting of a purified fibronectin matrix in Medium 199 supplemented with fetal bovine serum and endothelial cell growth factor. Cell proliferation and expression of differentiated function (factor VIII:Ag, tissue plasminogen activator and urokinase-like plasminogen activator) continues until approximately 50 to 60 cummulative population doublings (CPD) when HUV-EC senescence dominates; an observation consistent with the Hayflick model of in vitro senescence. However, HUV-EC populations can organize into viable, tubular, three dimensional structures in vitro when placed in an environment which minimizes HUV-EC proliferation. The mechanism of HUV-EC organization involves the constructive modification of extracellular matrix-derived fibronectin by plasmin. The process of HUV-EC organization represents a stable form of HUV-EC non-terminal differentiation since single cell suspensions derived from organized populations can revert to the proliferative state when subjected to the GFRE. The ability of HUV-EC populations to organize independent of the CPD level under culture conditions which minimize HUV-EC proliferation emphasizes the reciprocity between HUV-EC growth and differentiation. These observations have led me to seriously question the dogma of in vitro senescence by asking: do organized HUV-EC populations remember their in vitro age? The organizational behavior of HUV-EC populations presents a unique opportunity to address the concept of revitalization in vitro, a concept defined by the restoration of high proliferative potential at advanced CPD levels. Furthermore, this system will establish a strong relation between in vitro growth, differentiation and revitalization as an extension of the Hayflick model.